The invention relates in general to the opto-mechanical structure of semiconductor light sources. Especially the invention relates to the design of the encapsulation of such a light source for achieving optimal radiation characteristics.
Semiconductor light sources or LEDs (Light Emitting Diodes) are widely used for lighting in all kinds of electrical appliances. Their advantages include small size, low power consumption and very long service life.
In many cases it is necessary to use a limited number of light sources to illuminate a relatively large surface; usually there is also the requirement of as even a radiation intensity over the whole surface as possible. A typical example is the case of providing a backlight to an LCD or Liquid Crystal Display. FIG. 1 is a schematic cross section through a known LCD backlight arrangement. The eye of the viewer is supposed to be at the top, looking down essentially along the normal of the surface of an LCD element 101. Below the LCD element there is a lightguide 102, which is an approximately planar element the purpose of which is partly to allow light to propagate in the horizontal direction and partly to refract it upwards through the LCD element. The origin of the light is a semiconductor light source 103 or usually a number of them distributed around the circumference of the lightguide 102.
The performance of the arrangement according to FIG. 1 is evaluated in terms of the distribution of illumination observed by the user. The more even the illumination over the whole area of the LCD element, the better the backlight arrangement. FIG. 2 illustrates some known imperfections in the illumination pattern viewed by a user. The light emitted by the light sources 201, 202 and 203 may not reach far enough in the lightguide to provide illumination to the far end of the LCD element, giving rise to a dark zone 204 at the far end. On the other hand, a significant amount of light may be refracted through the LCD element in the immediate vicinity of the light sources, producing bright spots 205, 206 and 207. Additionally the radiation lobes of the light sources may not be wide enough, resulting in dark corners 208 and 209 and dark wedges 210 and 211 between the light sources.
The general developmental trend in portable electronic devices is towards even further miniaturization. On one hand this requires further reduction in the physical size of the components, meaning for example that the lightguide 102 illustrated in FIG. 1 should be made thinner. On the other hand the miniaturization sets tighter and tighter requirements to the power consumption of the devices, because a small device needs a small battery the capacity of which is limited. These requirements have made it more difficult to evenly illuminate a planar area like an LCD. If the lightguide is made thinner the relative amount of light coupled into it tends to get smaller, which has a general detrimental effect on the illumination. The requirement for further reductions in power consumption would suggest using a smaller number of light sources around the area to be illuminated, which naturally makes it more difficult to distribute the generated light evenly.
It is an object of the present invention to provide an improved lighting arrangement for generating a relatively even distribution of illumination over an essentially planar surface. It is a further object of the invention to provide such a lighting arrangement that would consume a relatively low amount of power. An additional object of the invention is to provided a lighting arrangement of the above-described kind which is advantageous in terms of applicability to automated mass production.
The objects of the invention are achieved by providing a novel design for the transparent substance encapsulating each semiconductor light source.
It is characteristic to the light source arrangement according to the invention that an encapsulated semiconductor light source comprises a semiconductor chip encapsulated in an encapsulation block that
comprises a curved surface facing the edge of an essentially planar light guide,
has a D-shaped cross section in a plane perpendicular to the plane of the essentially planar lightguide, the arcuate part of the D corresponding to said curved surface and
extends to a predetermined distance in an extension direction locally perpendicular to the plane of said D-shaped cross section.
According to the invention, the light generated in the semiconductor chip that constitutes the actual light source is collimated to a generally fan-like shape by using a transparent encapsulation around the semiconductor chip and designing it to a form that is basically an extruded D. The distance between the surface of the semiconductor light source and that of the surrounding encapsulation material in the principal direction of radiation is larger than the radius of curvature of the D-shaped surface. The encapsulated semiconductor light source is arranged into the close vicinity of a planar lightguide.
A study of the optical characteristics of the light source arrangement according to the invention shows that the fan-shaped distribution -of radiated light allows an effective coupling of radiated light into thinner lightguides than with prior art structures. Also the illumination pattern observed from a direction normal to the plane of the lightguide is more even. These advantages of the invention allow for the designer to select a suitable compromise between reduction in thickness and power consumption on one hand and enhanced illumination on the other. In other words, if the physical size and power consumption are the same as previously except for the novel design of the encapsulation, more effective illumination is achieved, and if the previously determined level of illumination is deemed sufficient, the number and size of separate components may be reduced.